Web Services (learning), how to call web method without [ScriptService]

This thread is more about learning, actually I have a web service which just returns string. I have been calling this service from code behind and updating lable with the string received from the web service.
Now I want to call web service using JavaScript. As far as I know, I have to add <asp:ScriptManager..../>. Second I have to add [ScriptService] to the web service.
But I don't wat to add <asp:ScriptManager..../> and [ScriptService].
Is there any way to call web service using javascript without adding <asp:ScriptManager..../> and [ScriptService]


Also, I am kind of lost in SOAP, JSON. I am not getting where I actually use SOAP, JSON. Using <asp:ScripManager../> and [ScriptService], I actually didn't see use of SOAP, JSON.
Can somebody shed some light on it.
...